1. Field of the Invention
The present invention relates to a gamma corrector which applies gamma correction to an image signal and an image display device such as a display device for a computer.
2. Description of the Related Art
FIG. 1 is a schematic block diagram showing a structure of a display device of the related art for a computer. The display device shown in FIG. 1 for a computer includes an image signal processing circuit 101, a deflection processing circuit 102 and a cathode ray tube 103 (called CRT in the followings). An image signal consisting of color signals SR, SG and SB (these three primary color signals are collectively called signal Si in the followings) of red (R), green (G) and blue (B), respectively, is inputted into the image signal processing circuit 101. A horizontal sync signal, Sh, and a vertical sync signal, Sv, are inputted into the deflection processing circuit 102. An output signal from the image signal processing circuit 101 and an output signal from the deflection processing circuit 102 are inputted into the CRT 103. The CRT 103 has an electron gun which includes cathodes for each color, R, G and B, and a deflection yoke for deflecting an electron beam emitted from the cathode to the vertical and horizontal directions (both the electron gun and the deflection yoke are not shown).
The image signal processing circuit 101 includes a preamplifier 105 and a main amplifier 106. The preamplifier 105 amplifies and outputs the inputted signal Si, and the main amplifier 106 further amplifies and outputs the output from the preamplifier 105. The preamplifier 105 and the main amplifier 106 are equipped with a separate circuit, not shown, for each of color signals SR, SG and SB of R, G and B, respectively. The deflection processing circuit 102 controls field generation caused by the deflection yoke, not shown, in the CRT 103 and deflects the electron beam emitted from the electron gun, not shown, to the vertical or horizontal direction, based on the inputted horizontal and vertical sync signals (Sh and Sv.)
The display device is able to display an image as follows. The CRT 103 generates an electron beam based on the output signals from the image signal processing circuit 101 and the deflection processing circuit 102 and then irradiates the generated electron beam on a fluorescent surface, not shown, to display an image.
The display device such as the above-described display device for a computer also usually includes a gamma corrector for correcting brightness of the image displayed on the CRT 103. The gamma corrector may be placed in front of the preamplifier 105 in the image signal processing circuit 101. The gamma corrector for the display device of the related art for a computer performs the one-point gamma correction, where a gain of an input/output characteristic of the image signal is varied at one point, which generally corresponds to an intermediate brightness. However, during the one-point gamma correction, when the signal level of the intermediate brightness at one point to be varied is turned up, the signal levels at other points which should not be varied, a signal level corresponding to gray, for example, is also turned up to some extent. This causes deterioration of image quality; a dark part in an image appears unclear, for example.
On the other hand, the gamma corrector in a display device for a television performs the gamma correction in which an input/output characteristic of an image signal traces a quadric curve, for example. However, the gamma corrector, whereby the input/output characteristic traces the quadric curve, requires considerable numbers of elements in the circuit and thus a frequency characteristic deteriorates due to a parasitic capacity caused by using a PNP transistor. In addition, the display device for a computer usually includes an image signal with a higher frequency (at least 100 MHz, for example) than that in the display device for a television. Thus, the gamma correction with the quadric curve cannot provide the sufficient frequency characteristic required for the display device for a computer, which deals with an image signal with a high frequency.
The invention is presented to solve these problems. An object of the invention is to provide a gamma corrector and an image display device that can perform the optimum gamma correction for an image display device such as a display device for a computer.
A gamma corrector according to the present invention inputs an image signal to a plurality of differential amplifiers which are connected in parallel each other and mixes a plurality of output signals which are outputted from the differential amplifiers to output an image signal to which the gamma correction is applied, each of the differential amplifiers amplifying and outputting the inputted image signal with a different gain characteristic so that the gain characteristic of the signal obtained by mixing the output signals varies at two inflection points.
An image display device according to the present invention comprises a gamma corrector which inputs an image signal to a plurality of differential amplifiers which are connected in parallel to each other and mixes a plurality of output signals which are outputted from the differential amplifiers to output an image signal to which the gamma correction is applied, wherein each of the differential amplifiers amplifies and outputs the inputted image signal with a different gain characteristic so that the gain characteristic of the signal obtained by mixing the output signals varies at two inflection points.
In a gamma corrector or an image display device of the present invention, each of the differential amplifiers amplifies and outputs an input image signal with a different gain characteristic so that the gain characteristic of the signal obtained by mixing the signals outputted from the differential amplifiers varies at two inflection points.
Other and further objects, features and advantages of the invention will appear more fully from the following description.